1. Field of the Invention
The present invention generally relates to the art of optoelectronic photosensors, and more specifically to a photosensor readout detector with increased dynamic range and immunity to anomalous high intensity signals.
2. Description of the Related Art
A photon-counting detector for a Long Wave Infrared (LWIR) or optical photosensor estimates the incident photon rate during sample intervals .DELTA.t by generating a voltage proportional to the number of photons collected by the photosensor during .DELTA.t. This is performed by an integrating circuit including a capacitor which charges with a current proportional to the instantaneous photon rate. The capacitor is discharged and the output of the integrating circuit thereby reset to zero at the end of each sample interval, where .DELTA.t is the reciprocal of the Nyquist frequency of the process being monitored. An example of such a readout is found in U.S. Pat. No. 4,929,913, entitled "GAAS FOCAL PLANE ARRAY READOUT", issued May 29, 1990 to R. Sato et al.
The conventional detector in which the capacitor is discharged at fixed time intervals is known as an "integrate-and-dump" readout, and suffers from several disadvantages. For example, the dynamic range of the readout must be designed to accommodate the largest anticipated signal, which implies either a limited dynamic range capability or high power requirement. Modifications for extending the dynamic range, such as gain switching or a split dynamic range photosensor are complicated and expensive to implement.
Another disadvantage is the switching noise which is generated each time the integrating capacitor is discharged. The readout is also sensitive to "gamma" events caused by natural and nuclear radiation, which create anomalous high intensity, short duration noise spikes in the photosensor signal. A very short gamma event can contaminate an entire integration sample, which can cause a significant reduction in signal-to-noise ratio (SNR) in a high gamma rate environment.